4 Vibrational Spectroscopy Umesh P. Agarwal and Rajai H. Atalla CONTENTS Introduction............................................................................................................ 104 Vibrational Spectroscopy .................................................................................. 104 Application to Lignin ........................................................................................ 105 Raman Spectroscopy.............................................................................................. 107 Background ....................................................................................................... 107 Instrumentation ................................................................................................. 108 Special Techniques and Effects......................................................................... 108 Micro Raman ................................................................................................ 108 Raman Imaging ............................................................................................ 108 Resonance and Preresonance Raman ........................................................... 109 ConjugationEffect........................................................................................ 109 Surface Enhanced Raman............................................................................. 109 Spectral Interpretation....................................................................................... 109 3100-2800 cm-1 .................................................................................................... 110 1800-1500 cm-1 ................................................................................................... 112 1500-1000 cm-1 ................................................................................................... 113 1000-350 cm-1 ..................................................................................................... 113 UV Resonance Raman Spectra ......................................... 113 Applications ...................................................................................................... 113 Lignin in Wood ............................................................................................. 113 Lignin in Mechanical Pulp and Paper .......................................................... 114 Residual Lignin in Chemical Pulp ............................................................... 115 Lignin in Other Lignocellulosics................................................................... 115 Commercial Lignins ...................................................................................... 115 Chemical Modification Reactions of Lignin ................................................. 116 Lignin Quantitation ....................................................................................... 116 Mid-Infrared Spectroscopy..................................................................................... 117 Background ........................................................................................................ 117 Instrumentation .................................................................................................. 118 Special Techniques/Interfaces............................................................... 118 Diffuse Reflectance ....................................................................................... 118 Attenuated Total Reflection, or Internal Reflectance .................................... 119 IR Microscopy ............................................................................................... 119 Chemical Imaging ........................................................................................ 119 2-D IR ........................................................................................................... 119 103 Lignin and Lignans: Advances in Chemistry Photoacoustic IR .......................................................................................... 120 Transient .................................................................................................. 120 Fiber Optic Mid-IR Spectroscopy................................................................ 120 Spectral Interpretation....................................................................................... 120 Applications ...................................................................................................... 121 Lignin in Wood ............................................................................................. 121 Lignin in Mechanical Pulps.......................................................................... 121 Residual Lignin in Chemical Pulp ............................................................... 123 Isolated Lignins ........................................................................................... 123 Lignin in Other Lignocellulosics.................................................................. 124 Lignin in Solution......................................................................................... 124 Lignin Quantitation ...................................................................................... 124 Near-Infrared Spectroscopy .................................................................................. 124 Background ....................................................................................................... 124 Instrumentation ................................................................................................. 125 Special Techniques and Interfaces .................................................................... 126 Applications ...................................................................................................... 126 Lignin in Wood and Leaves .......................................................................... 127 Residual Lignin in Chemical Pulp ............................................................... 128 Lignin in Other Lignocellulosics.................................................................. 128 Concluding Remarks.............................................................................................. 128 References.............................................................................................................. 129 INTRODUCTION Vibrational spectroscopy is an important tool in modern chemistry. In the past two decades, thanks to significant improvements in instrumentation and the develop- ment of new interpretive tools, it has become increasingly important for studies of lignin. This chapter presents the three important instrumental methods-Raman spectroscopy, infrared (IR) spectroscopy, and near-infrared (NIR) spectroscopy- and summarizes their contributions to analytical, mechanistic and structural studies of lignin. The conceptual frameworks used to interpret vibrational spectra are first described in the following section. V IBRATIONAL SPECTROSCOPY Vibrational spectra are of two types [1], infrared and Raman, and arise from two different types of energy exchanges between the molecules under study and electro- magnetic radiation. In infrared spectroscopy, a vibrational transition that involves a change in dipole moment results in absorption of an infrared photon. The energy of the absorbed photon is equal to the energy difference between the two vibrational states of the molecule. In Raman spectroscopy,the electromagnetic field induces a dipole moment in the molecule, with the result that an exchange of energy occurs simultaneously with the vibrational transition. The energy of the exciting photons is higher than the energy difference between the two vibrational states, and the exchange with the field results Vibrational Spectroscopy 105 in a scattered photon, shifted in frequency from the incident photon by an amount equal to the energy difference between the vibrational states. With both types of vibrational spectroscopy,distinctive spectra and facility in inter- pretation are possible because only vibrational transitions correspondingto changes in the vibrational quantum number of ±1 are allowed by the spectral selection rules. That is, ∆n = ±1, where n is the vibrational quantum number. Due to this, the frequencies observed are usually the fundamental frequencies. In addition, because of analogies between the mathematicaldescriptions of classical and quantum mechanical vibrating molecular systems, it is possible to rationalize many spectral observations by anal- ogy with classical vibrating systems that possess characteristic force constants and reduced masses. This rationalization has become the basis for systematizing much of the structural and chemical information derived from vibrational spectra. A useful exception to the primary selection rule is that the overtones and combina- tion bands associated with C-Hand O-Hstretching vibrations are, in fact, active in infrared absorption. Here ∆n = ±2 or ±3, which results in overtone and combination bands. The apparent violation of the selection rule arises because of anharmonicities in the potential energy function that govern the vibrations. These absorptions occur primarily in the near-infrared region because the selection rules are less rigorous in infrared absorption than in Raman scattering. These transitions are responsible for the spectral features in the NIR. Though the assignment of these transitions to par- ticular vibrational modes is not as easy, the spectral